Electric wave amplifier with feedback



ELECTRIC WAVE AMPLIFIER WITH FEEDBACK Filed Dec. .5, 1942 FIGJ I INVENTOR J. O. EDSON av ATTORNEY Patented Nov. 7, 1944 ELECTRIC WAVE AMPLIFIER WITH FEEDBACK James 0. Edson, Great Kills, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 5, 1942, Serial No. 467,945

8 Claims.

proved oscillator-amplifier and more particularly to reduce the deleterious effect that the oscillations generated tend to have on the amplifier and the transmission path in which it is interposed.

A further object is to reduc the tendency of the oscillations generated in an oscillator-amplifier to escape into the source of waves be amplified.

Still another object is to secure in an oscillator-amplifier the benefit of large negative feedback over a wide range of frequencies to beamplified and at the same time to prevent certain undesirable eflects heretofore incident to the generation of oscillations at a frequency or frequencies adjacent to the frequency range being amplified. In one aspect the present invention may be regarded as an improvement upon the amplifler-oscillator-modulator organization disclosed in United States patent to C. O. Mallinckrodt, No. 2,231,542, dated February 11, 1941.

In accordance with an embodiment of the present invention a wave amplifier having a large negative feedback over a range of frequencies to be amplified and having also positive or regenerative feedback for the generation of oscillations, is-

so arranged that the waves being fed back re-- generatively are injected into the negative feedback path.

The nature of the present invention and its various features, objects and advantages will appear more fully from a consideration of the embodiments illustrated in the drawing and now to be described. a

Fig. 1 illustrates one embodiment of the invention; and

Fig. 2 illustrates a modification thereof.

Referring to Fig. 1, there is shown schematically an electric circuit organization which, like those disclosed in the Mallinckrodt patent, supra, and in United States patent to J. H. Bollman, No. 2,231,558, dated February 11, 1941, is adapted for use at the sending end of a repeatered transmission line system having automatic repeater gain regulation of a type now known in the art. The function of the organization is several-fold: first, it constitutes a stabilized negative feedback amplifier having constant gain for the signals applied to its input terminals; second, it generates pilot oscillations for transmission over the lin concurrently with the signals; and, third, it

automatically varies the intensity of the generated oscillations in such manner that the average value of the total wave power delivered to the transmission line is substantially constant irrespective of the normal variations in the power content of the applied signals.

In general outline the circuit shown comprises a three-stage amplifier with input terminals l, output terminals 2, and a gain-reducing negative feedback path 6 extending from the output of the amplifier to the input thereof. Another path 1 extending from the output of the amplifier to a point within the negative feedback path 6 serves to provide suflicient positive feedback at a predetermined frequency that pilot oscillations are generated. A current-dependent element 8 funcof 12, 28 and 56 kilocycles, for example, which may be used at repeater stations for various regulating functions. It may be assumed further that the pilot oscillations generated within the amplifier have a frequency of kilocycles. It is to be noted that the intensity of these locally generated oscillations may range in the course of the normal operation of the system from substantially zero to a value comparable with the maximum signal output power for which the amplifier is designed.

The mu circuit of the amplifier and the negative feedback circuit 6 in this example are designedv in accordance with the teachings of United States patent to H. W. Bode, No. 2,123,178,

dated July 12, 1938, to yield maximum uniform negative feedback may be 45 decibels, for example. Feedback circuit 1 is designed to provide positive feedback from the output of the amplifier t0 the input thereof at the desired pilot frequency of 60 kilocycles. The current-dependent device 8 variably controls the amount of loss or attenuation in the positive feedback circuit, thus adjustably fixing the intensity of the generated oscillations, and the loss it introduces is in turn controlled in accordance with the power output of the amplifier.

Considering now the details of the circuit shown, the input terminals I are connected to the primary winding of an input transformer II 1 through a series inductance and shunting condenser the function of which is to give the transformer a good resistive impedance and substantially constant transmission from 12 to 60 kilocycles. The secondary winding of transformer I l is shunted by a condenser and by a resistance and condenser in series, the function of which is to bypass frequencies above the transmitted range and at the same time to give the complete network a good resistive impedance and substantially constant transmission from 12 to 60 kilocycles. The secondary is connected on the one side to the contype and are shown in association with respectivesecondary windings of a filament supply transformer.

The interstage coupling between pentodes 3 and 4 is of the impedance-condenser type with the elements proportioned for a substantially flat gain-frequency characteristic over the frequency range of interest. The interstage coupling between pentodes 4 and 5 is predominantly of the impedance-condenser type for the higher frequencies in the transmitted range and of the transformer-coupled type for lower frequencies, the elements again being proportioned for a. substantially fiat gain characteristic. Rectifier I is a substantially unilaterally conducting'device which, on the one side, is conductively connected to the control grid of pentode and, on the other, through ,a biasing battery I3 to the cath' ode of tube 5. The rectifier I0 is so poled as to permit current to fiow through it only toward the control grid of tube 5, and biasing battery I3 has such voltage that. the rectifier becomes conducting only when the signals being amplified become more negative than the cut-off voltage of pentode 5. The output of pentode 5 is deliveredto terminals 2 through an output transformer I5 the primary winding of which is tapped to afford a connection for the feedback circuit 6. Further details regarding this manner of securing a negative feedback connection will be found in United States patent .to E. H. Perkins, No. 2,210,001, dated August 6, 1940.

The negative feedback circuit 6 extends through the series elements of a T-network II, one winding 20 of a three-winding transformer I8, and a parallel resistor-condenser combination 22 to the high potential end of the impedance branch I2. The elements of this circuit are proportioned in the usual manner to provide negative feedback of the desired amount and of substantially the 'due to 60-kilocycle oscillations same value throughout the 12 to -kllocyc1e frequency range.

The positive feedback connections include a connection from the anode of pentode 5, through a blocking condenser and resistor in series, and a pair of inductively related coils 25, to ground. From the junction of the coils 25 the circuit 1 extends through a T-network including currentdependent device 8, and a winding 2| of the transformer I8, to ground. The third winding I9 of transformer I8 is shunted by a condenser 23 to form a low-loss circuit that is resonant at 60 kilocycles.

The 60-kilocycle waves being fed back through the circuit I are thus injected into the feedback circuit 6, and the latter provides a path for them back to the input of the amplifier. The elements making up the circuit 1 are so proportioned as to provide positive feedback from the output of the amplifier to the input for a frequency of 60 kilocycles, and the positive feedback so provided is made sufficiently greater than the negative feedback at 60 kilocycles which takes place. through circuit 6 that oscillations can be and are generated.

It is advantageous, because of the relatively low impedance of circuit I, to make windings 20 and 2| of comparatively few turns, four each for example, and Winding I9 of many turns, 340, for example, as in one instanceof practice, for by so doing the desired resonance can be obtained with a condenser of relatively small capacitance. The effect of the resonant circuit I9-23 is effectively to tune the windings 20 and 2| by virtue of the coupling between windings. Its effect on the negative feedback characteristic, in the band of signal frequencies, is largely offset by the resonant circuit associated with impedance branch I2.

An important feature of the circuit as herein disclosed relates to the magnitude of the 60-kilocycle voltage that appears in the input circuit of the amplifier, or more particularly, across the impedance branch I2. This voltage may be regarded as made up of two components, one due to transmission of 60-kilocycle oscillations through the negative feedback path and the other transmitted through the positive feedback path. Either of these two components alone may be hundreds of times as great as the difference between the two components. Bearing in. mind that either component alone may often be thousands of times as intense as any signal voltages present, one can appreciate the possibility that a substantial 60- kilocycle current may pass through the transformer II, due to the distributed capacitance of the windings thereof, and into the circuit to which the terminals I are connected. Thus, for example. these 60-kilocycle oscillations may enter through a preceding filter into a carrier telephone modulator with sufiicient intensity to cause distortion and unwanted modulation products throughout all of the carrier channels. In the present circuit, however, neither of the aforementioned components appears alone across the coupling impedance I2 but only the difference between the two components, and the difference voltage is so small that it can have no substantial effect on the connected circuit.

In order to vary the intensity of the -60-kilocycle oscillations in such relation to the normal variations in the intensity of the signals that the tion of the intensity of the current transmitted through it. There are various devices havin the required characteristic,-such as iron-cored coils and transformers, copper-oxide rectiflers and silicon-carbide resistors, but I prefer to use a thermosensitive resistance of such proportions that its temperature and resistance are largely controlled by the intensity of the electric current passing through it, such as the type 13? resistance lamp manufactured by Western Electric Company, Incorporated. The lamp has a positive temperature coefficient of resistance, and for present purposes it is therefore disposed in series in the positive feedback circuit I. Lamp 8 receives from the output of the amplifier a fixed proportion of the total power output. Hence as the total power output tends, for example, to rise with an increase in signal strength the resistance of the lamp increases, thereby decreasing the amount of positive feedback and the intensity of the 60-kilocycle oscillations. By suitable proportioning of the circuit elements in relation to the electrothermal characteristics of the lamp 8, the total power output of the amplifier may be maintained substantially constant.

The operation of the lamp is in some degree dependent on the ambient temperature. The adverse effect of ambient temperature variations is largely compensated, however, by a current-independent thermosensitive resistor 26. The latter is effectively shunted across the lamp network in the manner shown, and its temperature-resistance characteristic is so chosen as to complement the ambient temperature-resistance characteristic of lamp 8. The thermosensitive resistor 26, in conjunction with shuntin resistor 28, also affects the transient response of the oscillation circuit, tending to reduce the amount by which the oscillation intensity overshoots its'final value following-an abrupt change in the power input to terminals I. This effect appears, for example, when the oscillation intensity is required to change from zero to substantially its maximum value following cessation of an input pulse so strong as to stop the oscillation. The reduction in overshooting in such circumstances can be and is enhanced by designing transformer l8 to operate near saturation when the oscillation intensity is maximum.

The intensity of the oscillations tends to be affected also by any departure of the load impedance from a preassigned value. This effect is compensated by an auxiliary feedback path 9 which is connected, effectively in. parallel with feedback path 1, between the low potential end of the output transformer l and transformer winding 2|. If the impedance of the-load connected across output terminals 2 is, for example, lower than the preassigned value, the output voltage and also the amount of current fed .back' through path 1 will be lower than normal.

trated in Fig. 2, four impedance arms are disposed to form a bridge 30 with the secondary winding of input transformer ll connected across one diagonal, the feedback circuit 6 connected across the other diagonal, and the input to pentode 3 connected across one of. the impedance arms. Theoretically, the several impedance arms cuit bridge.

What is claimed is:

1. An electric wave amplifier comprising an input circuit, an output circuit, and a path connecting said input and output circuits for negative feedback over a broad band of frequencies, and means for generating oscillations at a frequency within the said broad band comprising said amplifier and a positive feedback path extending from said output circuit to a point within said negative feedback path, whereby the intensity of the oscillations effective in said input circuit is substantially less than the intensty of the oscillations fed back through either of said paths alone.

2. An electric w'ave amplifier comprising an input circuit and an output circuit for the waves to be amplified, a path embracing said amplifier for the negative feedbox of Waves within a range of frequencies, means including at least a portion of said amplifier for the generation of oscillations at a frequency within said range whereby oscillations comprising means for injecting oscillations from said generating means into said feedback path.

3. An amplifier having input and output terminals, a negative feedback path, and a positive feedback path for the generation of oscillations, said positive feedback path including means for injecting the generated oscillations traversing it into said negative feedback path at a point electrically distinct from the posterior end thereof.

4. A wave amplifying system having a gainreducing feedback path extending from a posterior point to an anterior point of said amplifying system, an oscillation-generating wave transmission loop comprising a positive feedback path and at least aportion of said amplifying system between said points whereby the said oscillations tend to be fed back through said gain-reducing feedback path to said anterior point, and means for injecting th generated oscillations into the said gain-reducing feedback path at a point electrically intermediate of the extremities of said gain-reducing feedback path.

5. A wave amplifying system having a gainre'ducing feedback path extending from a posterior point to an anterior point of said amplify ing system, anoscillation-generating wave transmission loop comprising a positive feedback path and at least a portion of said amplifying system between said points whereby th said oscillations tend to be fed back through said gain-reducing feedback path to said anterior point, means for injecting the generated oscillations into the said gain-reducing feedback path, and auxiliary positive feedback means responsive to changes in load plifier, and oscillation generating means comprising a path for regeneratively feeding back waves from a posterior point in said amplifier to said anterior point, said two paths comprising circuit means common to the two paths for completing them to said anterior point.

8. An electric wave amplifier having input and output-means for the waves to be amplified, a-

feedback path embracing said amplifier adapted for gain-reducing feedback over a range of frequencies including the waves to be amplified, and a path adapted for positive feedback at a frequency within said range comprising circuit means coupling an anterior point thereof with said first-mentioned feedback path in; phaseopposing relation with respect to said last-mentioned frequency. v

JAMES O. EDSON. 

